Facsimile transmitting and reproducing system, method and apparatus



Nov. 23, 1943. A. E: GERHARD ETAL ,8 FACS'IMILE TRANSMITTING AND REPRODUCING SYSTEM, METHOD AND APPARATUS Filed April 16, 1941 2 Sheets-Sheet 1 2 E IQYEETOE 5' Nov. 23, 1943 I I 5 R Q ETAL 2,334,818

FACSIMILE TRANSMITTING AND REPRODUCING SYSTEM, METHOD AND APPARATUS Filed April 16., 1941 2 Sheets-Sheet oouool Fun.

Patented Nov. 23, 1 943 FACSIMILE TRANSMITTING AND REPRO- DUCING, SYSTEM, DIETHOD AND APPA- RATUS AnthonyE. Gerhard, Los Angeles, Calif., and Everett G. Fraim, Hicksville, N. Y., assignors to Press Wireless, Inc., Chicago, 111., a. corporation oi. Delaware 1 Application April 16, 1941, Serial No. 388.742 @Claims. (Cl. 178-54 This invention relates to facsimile transmission systems and more especially to such systems which employ a transmission medium or link that is subject to fading, harmonic distortion and the like.

A principal object is to provide an improved system for transmitting facsimiles-and the like' over a radio channel, wherebythe effects of selective fading, harmonic distortion and the like are materially reduced, 1

A feature of the invention relates to an improved system for converting picture shades and the like into a frequency-modulated carrier the spectrum of which is confined to thealudio frequency range. 1

Another feature relates to an quency-modulating arrangement whereby the signal amplitude variations corresponding to picture shades are converted into a frequencymodulated audio frequency carrier by a, novel combination of oscillator tubes and a control tube by varying the impedance of a grid-controlled.

tube whose anode-cathode discharge path is effectively in shunt to an oscillatory circuit through a condenser of low capacity, whereby the variations of impedance of the control tube are refiected as variations in shunt capacitance across said oscillatory circuit. 7

Another feature relates to an improved converter for translating the output of a facsimile transmitting machine of the type producing picture signals in the form of an amplitude-modulated audio frequency carrier, into a frequencymodulated carrier limited to the audio frequency range. I

Another feature relates to a receiver for facsimile systems wherein the subject matter is improved fre-' Other features and advantages not specifically enumerated will be apparent after a consideration of the following detailed description and the transmitted as a frequency-modulated audio freappended claims.

Referring to the drawings which by way of example shows one preferred embodiment,

Fig. 1 is a schematic wiring diagram of the transmission end of a facsimile system embodyring features of the invention.

Fig. 2 is a schematic wiring diagram of the receiving end of a facsimile system embodying fea- I tures of the invention,

Referring to Fig. 1, the numeral i represents diagrammatically any well-known form of facsimile transmitting machine whereby the shades or tone values of successive elemental areas of the subject matter are scanned and translated into a corresponding amplitude-modulated audio frequency carrier, for example, a carrier of 1800 C. P. S. Reference may be had to U. S. Patent No. 2,015,742 for a typical machine of this type. The output of machine I is applied through transformer2 across the potentiometer resistance 3, the adjustable arm 4 of which is connected to grid 5 of amplifier tube 6. The potentiometer is also connected through condenser l to the cathode 8. Grid 5 is negatively biassed with respect to the cathode by the IR drop caused by the plate current flow through resistor 9; and a suitable series resistance In is connected between potentiometer 3 and ground. The steady high voltage potential for plate Ii is applied over conductor l2, series resistor l3 and transformer primary I4. Conductor I2 is supplied with'high voltage D. C, from any well known source consisting for example of A. C. mains Ma; step-up transformer I5; full wave rectifier l6; filter l1, l8, I9, 20, and any well-known voltage stabilizer 2|.

The amplitude-modulated A. C. .wave from transformer I4 is applied to a frequency doubling arrangement comprising a twin triode tube 22, the. inputcircuits between the grids 23, 24, and cathodes 25, 26, being connected in balanced relation while the plates are connected in phase to the primary winding of transformer 21. The

output of tube 22 therefore consists of a 3600 C. P. S. wave which is amplitude-modulated in accordance with the original picture signals from machine I. This wave is then impressed upon a full wave rectifier tube28 in theoutput circuit of which is connected a suitable filter comprising elements 29, 30,-3l, 32, whereby substantially all frequency flutter is eliminated and there is developed across the potentiometer resistance 31, a. D. C. voltage of varying amplitude The rectified signal from tube, 28 is applied to control a frequency modulator comprising a variable-frequency oscillator tube 33 and a fixed-- frequency oscillator tube 34 and a control tube 35. whose control grid 36 is connected to the ad justable arm of potentiometer 31 through a suitable bias battery 39 whereby grid 36 is negatively biassed with respect to cathode 39. Plate 40 is connected to ground, as is the low potential end of potentiometer 31. return circuit of tube 36 is completed through the control-grid to cathode circuit of tube 33 and includes the grid leak resistor 4| and the lower section of oscillator coil 42. Coil 42 is shunted by an adjustable tuning condenser 43 and this combination of coil and condenser is connected to the control grid 44, through a very low capacity condenser 45, for example of about .0001 mfd. It will be seen therefore that the oscillatory circuit formed by 42' and 43 is effectively shunted by a circuit consisting of the condenser 45, resistance 4l' and the cathode-to-plate discharge path of tube 35. Consequently, the fre 'quency of oscillation of tube 33 is determined in part by the elements 42, 43, as well as by the effective shunt capacitance of condenser 45 which in turn is controlled by tube 35. It will be noted that while the plate 40 of the control tube 35 is connected to ground, it is effectively at positive potential with respect to the cathode 39 when the tube 33 is in an oscillating state since in that condition a grid current flow exists between the grid 44 and the cathode 46 is of such 7 a polarity as to render the grid 44 and consequently the cathode 39 negative with "respect to ground. This is a D. C. potential difference because of the rectifying effect between the cathode 36 and grid 44.

Tube 33 is a so-called electron-coupled oscillator, for example a tube of the type 6J7 comprising 'a cathode 46, control-grid 44, shield grid 41, suppressor grid 48 and plate 49. The upper section of coil 42 is connected between 44 and 46 while the lower section of coil 42 is connected between 46 and 41, thus generating oscillations by feed-back action. The elements 44, 46 and 41, form in effect a triode oscillator of which the element 41 is in effect the anode, the plate 49 acting as a collector electrode for the electrons passed by grid 41. .The oscillator 34 is similar to oscillator 33 except that the feedback coil 50 is shunted by a fixed condenser 5i and the control grid 52 'is returned directly to round through the leak resistor 53. In both oscillators the respective suppressor grids may be connected directly to the associated cathodes. The controlled oscillator 33 is designed and. set so that variations of frequency can be eflected over a range of 1000 cycles corresponding respectively to the range of tone values or shades in the subject matter being scanned by themachine i. Thus oscillator 33 by means of potentiometer 31 and bias battery 33 can be adjusted so that for white shades there is generated a frequency of 265.5 k. c., and for black there is generated a frequency of 266.5 k. c. "The oscillator 34 may generate a fixed frequency of 262.0 k. c. The outputs of the two oscillators are applied to a common mixing tube 54 which may be of the type 6L1. The variable frequency voltages developed across resistor 55 are applied through coupling condenser 56 to the first con- The tube 35 may be of the triode type,

The grid-cathode,

corresponding to the shades of the original sub- 1 ject matter scanned by the machine I,

trol grid -51 of tube 54, the said game... normally negatively biassed by cathode resistor 58;

a suitable leak resistor 59 being provided. Likewise, the fixed frequency voltage deveioped across resistor 60 is appliedfthrough coupling condenser 6! to the second controlgrid 62. A suitable steady positive/potential is applied. to the "anodegrid" 63 by means of conductor 64. The fourth grid 65 is connected directly to the grid 51. By the well-known action of this type of tube, there is developed across the plate load resistor 66 voltages whose frequency is determined by the difference frequency between the oscillators 33 and 34. In -other words, under the above assumed conditions, there is developed an audio frequency band of from 3500 to 4500 C. P. S. This variable frequency signal is applied to the output transformer 61 through a radio frequency choke coi1'68 and coupling condenser 69 of the order of .005 mfd. The audio frequency modulations may then be applied to controlthe amplitude modulation of any well-known form of radio transmitter'representedschematically by the'nur the amplifier.

meral 10. It will be observed therefore, that the signals representing the shade values are in the form of audio frequency modulations having a frequency spectrum between 3500 and 4500 C. P. 8., so that the uppermost signal frequency,

namely 4500 C. P. C.-is less than the second har- -monic of the lowermost signal frequency, namely, 3500 C. P. S.

At the'receiving station, the received radio wave is detected and amplified by any wellknown "form of radio receiver 11 whereby the I 3500-4500 audiorfrequency spectrum is applied to the coupling transformer 12. 7 Part of the output of transformer 12 is fed to a volume controlled amplifier tube 13, for example of the type 6L7;

, and partof the output of transformer 12 is applied to the volume control or.regulator tube 14 which may be of the type 6R7. For the purpose of controlling the output levels, the first control grid {15 of tube 13 is connected to a variable tap 16 on the potentiometer-resistance 11. Likewise, the control grid 13 of theregulator tube 74 is connected to an adjustableitap' 19.- "The tube 14 is of the delayed action ,type'jand does not start functioningt until fa-predetermined minimum input level, e. 'g. '-'--12 db. is reached, then the regulator tube 14 takes fcontrol and holds the output of tube 13 constant to within approximately 0.5 db. until a level of 2 db. is reached corresponding to the beginning ofoverloading of The steady potentiais'for the plate and for the plate 8| are applied over conductor 82 from the high voltage D. C. supply unit which may be imilar to that used at the transmitter comprising A. C. mains-83; step-up transformer 64; full wave rectifier 85; filter '86, 81, 88, 89, and voltage stabilizer In accordance with the well-known operation of the type 6R7 tube, that is tube 14, as the plate current increases it applies a corresponding increased potential to the auxiliary anodes 9| whereby a corresponding biassing potential isdeveloped across the cathode resistor 92 thus controlling the current to plate 80 and therefore determining the potential of the second control grid 93 of tube 13. suitable combination of resistors 94, 95 and condenser 96, are provided having a predetermined time constant to determine the extent of delay of action of the regulator tube 14 on grid 93.

The plate or output circuit of tube 13 is connected through a resonant circuit consisting of 'potentials being derived over conductor the series condensers 91, 98, resistor 99 and a parallel winding I of the coupling transformer IN and shunt condenser I03. The elements of the combination 91 to I03 are chosen in magnitude so as to provide a resonant circuit which is resonant at 3500 C. P. S. and which has a sloping characteristic on either side of 3500 cycles with a maximum attenuation at 2500 and 4500 cycles respectively. Consequently the signals that are applied to the control grid I04 of a succeeding amplifier tube I05 are confined to the range between 2500 and 4500 C. P. S. Consequently, if there has been any harmonic distortion in the form of frequency doubling at any point between the transmitter and the receiver 1I, it is effectively eliminated since the second harmonic of the lowermost signal frequency of 3500 C. P. S. is not passed to the amplifier tube I05. Tube I05 may be of the type 6N7 for amplifying the 25004500 spectrum applied to the control grid thereof. The output of tube I05 is then passed through a frequency doubling tube I06 of the type 6N7 similar to the tube 22 at the transmitter. The doubled frequency signals are then applied to the full wave rectifier tube I01, the circuit of which includes a suitable filter comprising elements I08, I09, IIO, III, H3, H4, H5, H6, whereby substantially all frequency flutter is eliminated and the voltage developed across the potentiometer resistor II1 consists of D. C. signals with amplitude variations substantially identical with the amplitude variations from the tube 23 at the transmitter. These D. C. amplitude variations which represent the original picture shades at the transmitter machine I, can be used to control the reproducing amplitude-modulated audio frequency carrier. For this purpose, there is provided an audio fre quency oscillator tube II9 which may be of the type 6J7 having a feed-back oscillator coil I connected between the control grid I2I and the cathode I22, another section of this coil being connected between the cathode I22 and the anode-grid I23. The suppressor grid I24 may be connected directly to the cathode. A tuning condenser I25a is connected across the oscillatory circuit to control the oscillation frequency, for example 1800 C. P. S. The operating potentials for the anode-grid I23 and the collector plate I261: are derived over conductor 82. The 1800 cycle signal from oscillator I I9 is applied in pushpull relation to the input grids I25 of the mixer tubes I26, I21, which may be of the type 6L7. The D. C. signals representing the picture shades are tapped off from the potentiometer H1 and are applied in parallel to the second control grids I28 of tubes I26, I21.

through the output resistor I29, the steady plate I30.

Consequently there is developed across resistor I29 an 1800 cycle signal having amplitude modulations corresponding to the original picture shades. This amplitude modulated wave is then The plates of the tubes I26 and I21 are connected in balanced relation passed through a suitable amplifier m which may be of the type 6N7 preferably connected to operate as afclass A output amplifier to raise the signal to the desired level for controlling the 5 reproducing element of the facsimile machine I have found that with the foregoing arrangement, a satisfactory picture having a wide range of tone values can be transmitted over long distances either by wire or by radio and the effects of fading and harmonic distortion such as fre-- quency doubling and the like in the transmitting link, are substantially eliminated. While specific apparatus and circuits have been disclosed for achieving the objects of the invention, it will be understood that various changes and modifications may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. A facsimile transmitter comprising a machine for translating tone values into a corresponding modulated audio frequency carrier. means to double the frequency of said carrier, means to rectify said doubled frequency carrier 25 and to derive therefrom substantially pure D. C.

signals corresponding to the original tone values,

a variable frequency oscillator, a control tube for said oscillator for controlling the frequency thereof, and means to vary the conductivity of 30 said control tube by said D. C. signals.

2. A facsimile transmitter according to claim 1 in which the variable frequency signals are confined to an audio frequency spectrum wherein the frequency corresponding to one extreme range of tone value is less than the second harmonic of the lowermost frequency corresponding to the opposite extreme range of tone value.

3. In a facsimile system, means to receive. over a transmission channel which is subject to o harmonic distortion, facsimile signals in the form of a frequency modulated carrier wherein the carrier frequency is within the same order of frequency as the modulations and which modulations represent the tone values of the original subject-matter to be transmitted, means to detect said modulations, means including a resonant network to eliminate therefrom all parasitic frequencies which area second or higher harmonic of any frequency within the said modulation range, means to double the frequency of said detected modulations, and a rectifier and filter for converting the doubled frequencies into substantially pureD. C. variableamplitude signals. and a facsimile reproducer controlled by said D. C. signals.

4. A facsimile receiver comprising means to receive and detect frequency modulations in an audio frequency spectrum, the limits of which correspond to the tone value limits of a picture to be received, means to convert said audio frequency spectrum into corresponding -D. C. amplitude variations, the last-mentioned means comprising a network which has a variable attenuation effect on said spectrum in accordance with the individual frequencies thereof, a frequency doubler for said spectrum, and a rectifier and i-fllter for converting the doubled frequencies into -substantially pure D. C. variable amplitude signals, and a facsimile reproducer controlled by 

